Lead-acid batteries are fitted in practically every vehicle, but the type of chemical-basis used in the cells has advanced considerably. A typical lead-acid battery is known as a "flooded-cell battery," because it contains fluid. However absorbed glass mat (AGM) and gel lead-acid batteries are a safer option, especially if your battery is susceptible to movement, such as on a marine vessel or a portable battery booster, where spillage or leakage is more likely to occur.
Gel batteries use a silicon-based agent to thicken the electrolyte in the battery cells which forms into a gel. AGM batteries use a silicon glass fibre compound which is placed between the lead plates and the casing of the battery. The chemical basis of both battery types makes them spill proof if the battery casing gets cracked or damaged.
AGM batteries are easier to charge. There is no requirement to purchase a different charging unit as you can use a regular flooded-cell battery charger. However, gel batteries need to be charged at a slow rate. If you boost charge a gel battery it is likely it will be permanently damaged. Additionally, the voltage rate used to charge a gel battery needs to be about 20 per cent lower than regular lead-acid or AGM batteries. Too much voltage and the battery gets hot resulting in holes appearing in the gel causing a loss of battery capacity.
If the temperature around a gel battery drops much below freezing level, and remains so for a while, the gel can freeze causing expansion and the battery casing will crack. AGM batteries use a solid compound and therefore freezing temperatures have no bad effects on the battery.
AGM batteries have a very low self-discharge rate which equates to about 1 to 3 per cent a month. This gives an AGM battery a particularly long shelf life and also means that if you leave your battery unused for some time you can be certain it still retains a reasonable charge. Gel batteries don't self-discharge at the rate of a regular lead-acid battery, but the rate is higher than an AGM battery.
Recharging Dead Batteries
You may have a problem getting a gel battery to take a full charge, if it has remained fully discharged for longer than about 30 days. A gel battery can lose its ability to charge rendering it useless. An AGM battery that has remained fully discharged for 30 days is still able to take a 95 per cent charge and will still take a reasonable charge if left discharged for longer.